This invention relates to an ultrasonic probe used in an ultrasonic diagnostic apparatus for medical use.
It is a recent trend to use ultrasonic probes made of compound piezoelectric materials in the field of ultrasonic diagnostic apparatus for medical use and the like.
Such ultrasonic probe using compound piezoelectric materials has been conventionally known, including, for example, the construction mentioned in Proc. IEEE Ultrasonics Symposium, 1984, by A. A. Shaulov et al.
Referring now to FIGS. 1 and 2, the conventional array type ultrasonic probe using compound piezoelectric materials is described below.
FIG. 1 is a perspective view showing the essential parts, and FIG. 2 is a magnified sectional view of the portion of taking out lead wires thereof. Numeral 101 denotes a piezoelectric material, being composed of a piezoelectric ceramic 102 stretched in one-dimensional direction, and an organic polymer material 103 stretched in three-dimensional directions surrounding said piezoelectric ceramic 102. On one side of this compound piezoelectric material 101, a plurality of compounds 104 are disposed by vacuum evaporation by way of mask or the like, while a common electrode 105 is installed on the opposite side of the compound piezoelectric material 101 by vacuum evaporation, and an acoustic matching layer 106 made of epoxy resin or the like is adhered to the common electrode 105 side (the sound wave radiation side) in order to lead the sound waves efficiently to the subject to be examined. From the individual electrodes 104, lead wires 108 are taken out by wire bonding or similar means, and are connected to electric terminals (not shown).
Thus, by applying electrode signals controlled from outside into the electrodes 104, 105 at both sides of the compound piezoelectric material 101, ultrasonic waves can be radiated from the acoustic matching layer 106 side.
In this compound piezoelectric material 101, the piezoelectric ceramic 102 is made of PZT, and the organic high polymer material 103 may be made of epoxy resin or the like, and such compound piezoelectric material 101 can, in the electro-mechanical coupling coefficient, obtain a characteristic nearly equal to k.sub.33 of the electro-mechanical coupling coefficient of the piezoelectric ceramic 102, and may be lowered in the acoustic impedance as compared with that of piezoelectric ceramic alone (in the case of PZT compound, it is about 20 to 35.times.10.sup.5 g/cm.sup.2 .multidot.s). For example, if, by volume ratio, the piezoelectric ceramic 102 is used in an amount of 25%, and the organic polymer 103 of epoxy resin in an amount of 75%, the acoustic impedance is about 8.times.10.sup.5 g/cm.sup.2 .multidot.s, and its matching with the subject such as the human body (of which acoustic impedance is 1.5 to 1.8.times.10.sup.5 g/cm.sup.2 .multidot.s) is much better than in the case of piezoelectric ceramic alone, and only one layer of acoustic matching layer 106 is enough to improve the efficiency (sensitivity) of transmission and reception. Besides, since the piezoelectric ceramic 102 is one-dimensional direction only, leaks of sound waves to other elements (acoustic crosstalks) are few, which means that the orientation resolution is excellent.
Accordingly, the usefulness of ultrasonic probe using compound piezoelectric material 101 with a single acoustic matching layer 106 has been disclosed.
However, as in this conventional construction, when lead wires are taken out after forming an array of electrodes 104 by masking or other means on one side of the compound piezoelectric material 101, in spite of the advantage of high precision forming of electrodes 104 in an array form by employing the semiconductor manufacturing technology, it is practically rather difficult to take out lead wires 108 by wire bonding or other means after forming electrodes 104. That is, the compound piezoelectric material 101 is, as mentioned above, made of piezoelectric ceramic 102 and organic polymer 103 of epoxy resin, and this part is magnified in FIG. 2. More specifically, due to the difference in hardness and coefficient of thermal expansion between the piezoelectric ceramic 102 and organic polymer 103, the surface of the organic polymer 103 is not flat, but undulated, and it is hard to avoid this roughness. When electrodes 104 are formed on such undulated surface in a thickness of about 1 micron by vacuum evaporation or other means, the electrodes 104 are unevenly formed on the undulated surface of the compound piezoelectric material 101. It is extremely difficult to take out lead wires 108 by wire bonding uniformly and firmly on such non-flat surface. In particular, since wire bonding at the position of organic polymer 103 is difficult, it is required to take out lead wires 108 by wire bonding at the position of piezoelectric ceramic 102. However, at high frequency, the shape of the piezoelectric ceramic 102 becomes less than 70 microns, and wire bonding is barely possible. But when the lead wires 108 are taken out by wire bonding at the position of the electrodes 104 arranged in a plurality, the strength is lower and reliability is insufficient, or at higher frequency it is hard to take out lead wires 108.
As the mechanism of array type ultrasonic probe, meanwhile, the method of driving by sequentially scanning the transmission and reception signals to be applied to a plurality of array of electrodes 104, and the method of scanning by varing the running direction of the ultrasonic waves by driving, with a very slight time delay, the transmission and reception signal to be applied to a plurality of array of electrodes 104 are known. In these cases it is important that the sound waves may not propagate to the adjacent electrode parts when the arrayed electrodes 105 are driven, that is, the acoustic crosstalk be small, in the light of enhancing the orientation resolution. However, in such conventional structure, the acoustic matching layer 106 was not isolated from the plurality of electrodes 104, but was made of one continuous plate, and the unnecessary sound waves propagated to the adjacent electrode 104 parts from the acoustic matching layer 106 part, which caused deterioration of the orientation resolution.